In the past, it has been difficult to determine the precise location of holes in surfaces when performing quality control inspections. When conforming a part to the dimensions of a drawing, it is critical to determine the three axes coordinates of the center of a hole, such as a drilled or cast hole, as well as other points on the part with respect to a preselected, known reference point. Coordinate measuring machines are frequently used to perform the necessary measurements to determine these coordinates; however, when a hole is located in an oblique surface which is not parallel to two of the three orthogonal reference axes (commonly denominated X, Y, Z) or the hole is formed in the part at other than a right angle to the surface, it is extremely difficult to use these machines to achieve accurate measurements.
One such measuring machine tracks the position and travel of a probe supported on an arm for movement along the three axes. The probe has a tracing ball or electronic probe mounted on a shaft, and the user moves the ball around to read the coordinates of various surface points relative to a preselected reference point. Use of this machine to determine the location of a hole requires measurement of several points common to the interior of the hole and calculation of the hole's location therefrom. If, however, the hole is in an oblique surface or skewed relative to the surface, the measurements made by the probe provide coordinates of an ellipse instead of a circle, making it difficult to calculate a correct hole location. If the angle of the surface or hole is extreme relative to the orientation of the probe, it is sometimes impossible to even trace the hole with the tracing ball without the shaft of the probe contacting the part and producing nonsensical readings. While these problems can be eliminated by reorienting the part relative to the measuring machine, or vice versa, to position the surface with the hole parallel to two of the axes or the skewed hole parallel to one of the axes, this requires inconvenient, time-consuming and expensive movement of the part and re-indexing of the part to the reference point. Such a solution is impractical if the part has holes to be measured in many different oblique surfaces.
For a flat surface parallel to two of the three reference axes with the hole formed at a right angle relative to the surface, a conventional tooling ball may be used as an aid to determine the hole location. Such a tooling ball consists of a shaft insertable in the hole which holds the central longitudinal axis of the tool in alignment with the center line of the hole, a surface-engaging collar extending radially from the shaft, and a spherical ball attached to the shaft with its center point along the central axis of the tool. The shaft of the tooling ball is inserted into the hole, and the user takes several measurements of the ball's surface with the measuring machine. A computer, typically comprising part of the measuring machine, is used to compute the coordinates of the center point of the ball. Since the collar holds the ball, and hence its center point, at a known distance away from the surface, a simple adjustment to the coordinates of the center point of the ball by the amount the center point is held away from the surface will yield the coordinates of the center of the hole.
A conventional tooling ball, however, will not work on an oblique surface without reorienting the part and reindexing the machine, as discussed above. This is because the tooling ball produces only the coordinates of a point in space without any known reference to the hole. Unless that point is known to be at a predetermined distance away from the hole along one of the three reference axes, as in the situation discussed above, the coordinates of the hole location cannot be determined from one point. With a skewed hole, the collar will not sit flush against the surface, so the center point of the ball will be at an unknown distance away from the surface, preventing determination of the hole coordinates.
Another type of measuring machine uses a non-contact technique involving a camera. The camera is positioned relative to the part and uses images to determine the coordinates of points on the part. It is, of course, difficult to produce a true image of a hole, particularly a hole in an oblique surface.
It will therefore be appreciated that there has been a significant need for an inspection tool which may be conveniently used to determine the location of a hole in an oblique surface without reorientation of the part or measuring machine, or reindexing of the reference point. The inspection tool should also be usable with non-contact type measuring machines. The present invention fulfills this need, and further provides other related advantages.